This should probably be in the MLP thread.
I've read the original and Project Horizons, and read part of Pink Eyes but gave up because it sort of svcks.
The original sticks pretty close to FO3's story, just with everything ponified. There's plenty of original elements that aren't touched upon in the game to my knowledge (the elements of pre-war politics like the ministries, and of course all the magic). There's also plenty added in that doesn't quite match up to the game. I could be wrong, but I don't think there's a game equivalent to Canterlot and the pink cloud.
I'd actually say it's more grimdark than the game. Just because the rendered environmental gore in the game is fairly limited compared to what you can create in a story. The most you get from the game's raiders are some dismembered corpses and bloody bones.
Okay, this sounds like a completely hilarious concept, and even if it is told completely straight - neigh! (Heh) - especially if it is told completely straight, I'd probably laugh my ass off at the insanity of it all.
Regardless of how much you hate ponies, you'd have to be a pretty disturbed individual to laugh through 600,000 words of [censored], murder, torture, and death. I think you're underestimated exactly how grimdark it is

And Project Horizons is arguably
worse for that. There's a very disturbing few scenes, one that was so bad the author gave a warning and provided a link to skip it (I didn't, it wasn't
that bad but it wasn't censored).